Input tablets are classified by their operating principle. Examples of input tablets include: resistor sheet input tablets, conductive sheet input tablets, optical scanning input tablets, surface-wave transmission time detecting input tablets, and strain gauge input tablets.
Resistor sheet input tablets, among other types of input tablets, utilize the principle that the positional coordinates of the points of input on the resistor sheet are detectable from the corresponding ratios of the electric current or the electric potential. Thus, they are simple in structure and have a low cost of production. However, for successful use of resistor sheet input tablets, there must be a uniform distribution of the fixed sheet resistivity throughout the entire area of the resistor sheets.
The resistor sheets which are available on the market or are still under development are produced by a method which involves applying, to one side of a synthetic resin film, a dispersion of a conductive material such as carbon powder mixed with a thermosetting synthetic resin, i.e., carbon paste (or carbon ink) by screen printing or by the use of a roller. However, these resistor sheets lack the required uniform distribution of the conductive particles in the paste and the required uniform thickness of the coat applied to the substrate film. Thus, these resistor sheets fail to provide an exact correlation between the positional coordinates of the points of input and the corresponding ratios of the electric current or the electric potential. Hence, these resistor sheets are disadvantageous in that the reproduced images are inevitably distorted.
Japanese Patent Application (OPI) No. 97137/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), which teaches a solution to the above-described problem, discloses a resistor sheet comprising several oblong strips which are insulated from one another and arranged side by side to complete one sheet. However, this resistor sheet is disadvantageous in that it fails to receive delivered information on the boundaries of the adjoining oblong strips, it has a complicated structure and it is expensive to manufacture.
Japanese Patent Application (OPI) No. 51425/79 discloses an electric resistor fabric woven in the form of a sheet using electrically insulated yarns and electrically resistant wire as warps and wefts, respectively. However, this electric resistor fabric has not yet been accepted for actual use because it is disadvantageous in that it only offers limited resolution, does not solve the problem of the susceptibility of the meshes to deformation and requires unusually high pressure in operation.